Prompt fission product yields in the ${}^{238}\mathrm{U}(n,f)$ reaction

Background: Significant yield discrepancies (500%–600%) were reported recently between experimental results and predictions (from the gef model) and evaluations (from the JEFF-3.1.1 and ENDF/B-VII.1 libraries) for Mo and Sn fission-fragment yields in fast-neutron-induced reactions on ${}^{238}\mathrm{U}$ using $\gamma -\gamma -\gamma$ coincidence spectroscopy. The model and evaluations also predict Mo and Sn fragments that are on average $\sim 1$ to 2 neutrons richer than the experimental results.

Purpose: $\gamma -\gamma -\gamma$ coincidence spectroscopy favors detection of higher-multiplicity $\gamma$-ray cascades. An alternative approach is determining the fragment yields using single-$\gamma$-ray spectroscopy, as it was attempted here for selected cases where it was feasible. Advantages and drawbacks in both approaches need to be understood and potential systematic errors in the experimental results should be addressed using theoretical models.

Methods: Fast neutrons from the LANSCE WNR facility were used to induce fission on ${}^{238}\mathrm{U}$. The emitted $\gamma$ rays were measured with the GEANIE spectrometer.

Results: The yield of selected even-even fission fragments was determined. The selection was based on the ability to reliably determine excitation functions for the detected $\gamma$ rays.

Conclusions: Our single-$\gamma$-ray results provide better agreement between experiment and predictions and evaluations.

Figure 1

Cross-section values as a function of incident-neutron energy obtained for the 350.7 keV (solid circles) and the 769.2 keV (open squares) transitions of ${}^{106}\mathrm{Mo}$ and of ${}^{92}\mathrm{Kr}$, respectively, in the present work. The solid line is the total ${}^{238}\mathrm{U}(n,f)$ cross section from Ref. [36] divided (arbitrarily) by a factor of 100. The cross-section values obtained in the present work for the 974.1 keV transition (gray triangles) are also included: this is, most likely, the $2^{+}\to 0^{+}$ transition of ${}^{132}\mathrm{Te}$ [28], and due to the added time of flight (see discussion in text) from the deexcitation of the known $28.1\mu \mathrm{s}$ isomer [28] of ${}^{132}\mathrm{Te}$, the resulting excitation-function shape is “distorted” at low incident-neutron energies. The neutron-energy error bars correspond to TOF bin widths while the cross-section error bars are statistical.

Figure 2

Yields from Table 1 (solid symbols) plotted versus mass of the fragments. The yield values for ${}^{128,132}\mathrm{Sn}$ and ${}^{134}\mathrm{Te}$ are upper limits due to known isomers (see text). The black dash-dotted line is a Gaussian fit to the yields obtained for ${}^{104}\mathrm{Mo}$ and ${}^{106}\mathrm{Mo}$ in the present work. The experimental yields from Fig. 3 of Ref. [10], multiplied by the ${}^{238}\mathrm{U}(n,f)$, 460 mb [36] cross section at $E_n=1.72$ MeV, are included as open symbols. The solid lines are the evaluated product yields for a fission neutron spectrum as quoted in Ref. [52], the dashed lines are the Wahl systematics (CYFP parametrization) [54, 55] at $E_n=1.72$ MeV, and the dotted lines are the predictions by the gef code [53] at $E_n=1.72$ MeV, all normalized by multiplication by 460 mb.

Figure 3

Fission product yields for the 19 isotopes in Table 1 as deduced from $\gamma$-ray spectroscopy (GEANIE yields from the present work and those determined by Wilson et al. in Ref. [10]) and as calculated with the cgmf code using either a single $\gamma$-ray transition (dotted) or a double gate on the $4^{+}\to 2^{+}$ and $2^{+}\to 0^{+}$ transitions (dashed). The solid line indicates single-gate yields that have been corrected with the level ${\epsilon }_L$ and purity ${\epsilon }_P$ corrections (see text). Values from the evaluation in Ref. [52] (England and Rider) are also shown.